Mouse mammary tumor proviral (MMTV) genes are constitutively expressed in cells of the B lineage and can be regulated during B cell differentiation. One MMTV gene, the open reading frame (ORF) of the 3' long terminal repeat (LTR), has been shown to encode "endogenous superantigens". When expressed endogenously, superantigens delete T cells expressing appropriate V Beta elements. The 3' LTR ORF also encodes a protein(s) that is a transactivator. The function of the transactivator is no known, but it may be important in the viral life cycle and may contribute to the transforming potential of the virus. The experiments outlined in this proposal are designed to characterize the regulation of MMTV gene expression in cells of the B lineage, and to determine the function(s) of LTR-derived proteins. We will evaluate the regulation of superantigen expression in B lymphocytes, and determine at what level(s) the functional expression of superantigens is post- translationally regulated. We will determine the structural and cellular requirements for functional transactivation by the MMTV 3' LTR ORF, and determine if transactivation is a B cell restricted effect or requires association of the 3' LTR protein and class II molecules. We will assess the relationship between the proteins that are responsible for transactivation and superantigen activities. We will characterize the cellular genes that are transactivated in B cells, and determine the intermediary protein(s) through which transactivation is mediated. We will characterize the cis-regulatory sequences in the U3 region of the MMTV LTR which control constitutive and upregulated gene expression in LPS and IL-5 stimulated B cells, and determine of the proteins which bind these sequences are similar to other known transcriptional regulatory proteins. We will determine the role of the immune system in the MMTV life cycle, testing the hypothesis that interactions between superantigen reactive T cells and virally infected B cells are required for the horizontal transmission of the virus in infected mice. Finally, we will use transfected cells and transgenic mice constitutively expressing the 3' LTR ORF gene to determine if other protooncogenes cooperate with this MMTV encoded gene in cellular transformation. The studies proposed in this application will address issues concerning the biology of B lymphocytes, the interactions of viruses with cells of the immune system, and fundamental problems concerning the nature of transactivating proteins, transformation, and oncogenesis.